Preheat oven to 250°F. Line a baking sheet with parchment paper; set aside.
Melt butter in a medium saucepan over medium high heat. Stir in brown sugar, corn syrup, cinnamon and salt. Without stirring, bring to a boil for 4 minutes. Remove from heat and add vanilla and baking soda.
In a large bowl, combine popcorn and brown sugar mixture until evenly coated.
Spread popcorn onto prepared baking sheet in an even layer. Place into oven and bake for 45 minutes, stirring in 15 minute intervals. Drizzle with chocolate.